Update to newer posh-git functions and have revision number of working copy in the prompt

[JackDMF]

I figured it might be nice to update this extension to use the newer posh-git functions. Also I needed the revision number on the prompt (as in the current working copy state - changes with svn up).

Please review these changes :-)

[JeremySkinner]

Thanks for the contribution, but I'm not actually working on this project any longer. If you'd like to fork it and continue work, then please feel free, but I'm not personally intending on make any further amendments or changes.

[JackDMF]

Ok, so I'll keep it in my github repo. But thank you very much for this project! It makes life in powershell a lot easier :-)